Kintan asks in his blog as to what is the killer app of Web 2.0. My bet is on Email. Any software company (enterprise or otherwise) that can successfully change our email experience will be a huge winner. And to the naysayers out there- it has already happened once. Hotmail is not what email started out as in 90s. Its actually a web-based application that looked like email. For example, note that Hotmail-to-Hotmail email may never see SMTP- its actually a database (or file) centric web application.
It can be done again. But who? I know that Google is trying and that Microsoft has the incumbent's advantage and the incumbent's curse.